ANALYSIS OF THE STRUCTURE OF EMPTY AND PEPTIDE-LOADED MAJOR HISTOCOMPATIBILITY COMPLEX-MOLECULES AT THE CELL-SURFACE

We compared the conformation of empty and peptide-loaded class I major histocompatibility complex (MHC) molecules at the cell surface. Molec ular conformations were analyzed by fluorescence resonance energy tran sfer (FRET) between fluorescent-labeled Fab fragments bound to the alp ha 2 domain of the MHC heavy chain and fluorescent-labeled Fab fragmen ts bound to beta 2-microglobulin. No FRET was found between Fab fragme nts bound to empty H-2K(b), but FRET was detected when empty H-2K(b) m olecules were loaded with peptide. The magnitude of FRET depended on t he sequence of the peptide used. The results imply that empty H-2K(b) molecules are in a relatively extended conformation, and that this con formation becomes more compact when peptide is bound. These changes, w hich are reflected in peptide-dependent binding of monoclonal antibodi es, affect the surfaces of MHC molecules available for contact with T cell receptors and hence may influence T cell-receptor recognition of MHC molecules.